It's a day on which Lee Greenwood's "God Bless the USA" is probably second only to the National Anthem, but it seems Americans haven't been less "proud to be an American" since before Sept. 11, 2001.

A new Gallup poll shows 54 percent of Americans say they are "extremely proud" to be Americans. That number is down slightly from the mid-to-high 50s, where it stood for much of the last decade -- after its post-9/11 peak.

In addition, the 19 percent of Americans who express moderate (14 percent), "only a little" (4 percent) or no pride (1 percent) also hit a new 21st century high.

A major reason for less pride: millennials. No group broken down by Gallup had fewer people say they are extremely proud to be Americans. Just 43 percent of adults under 30 said this applies to them. (The definition of millennials also generally include those 31 to 35 years of age, though that's not broken out in Gallup's numbers.)

But it's also worth noting that the number of extremely proud Republicans has dropped -- from 74 percent in 2006, for example, to 68 percent today.

The GOP still remains the proudest group, while Democrats, at 47 percent, are one of the least-proud.

Here's the full breakdown: